1. Field of the Invention
The present invention relates to organic coated materials having excellent corrosion resistance and lacquer adhesion, and methods of economically making the same. In particular, the present invention relates to electroconductive substrate with an electrolytically polymerized coating film containing chromium formed thereon, and a method of making the same. More particularly, the present invention relates to steel or zinc-type plated steel substrate, suitable for vehicle bodies, having an electrolytically polymerized coating film containing chromium, and relates to a method of making the same.
2. Description of the Related Art
Application of various surface-treated steel sheets to vehicle bodies are increasing every year in response to the demand for high corrosion resistance. A zinc-type plated steel sheet is a typical example of a surface treated steel sheet.
However, steel sheets for vehicle bodies require more corrosion resistance at concealed and hemming portions at the inner sections of the body which are exposed to high humidity, because coating after body assembly is not always sufficient at such portions.
In response to such demand, organic composite coated steel sheets, of which a chromate layer and an organic polymer resin layer containing silica and having a thickness of several .mu.m or less are coated on a zinc-type-plated steel sheets, have been proposed, as disclosed in Japanese Unexamined Patent Publication Nos. 91-130,141 and 90-258,335, for example. Such organic composite coated steel sheets are widely used for the interior of the body, because these exhibit excellent corrosion resistance even through noncoated after body assembly.
Cost reduction has been continuously required in the vehicle industries. Since the organic composite coated steel sheets are made with use of a chromate layer coating step and an organic layer coating step after plating, each step needs a coater station and an oven for baking, resulting in increased facility costs and operation costs. Further, mechanical properties of organic composite coated steel sheets may worsen after baking.
Electrodeposition and electrolytic polymerization can provide an organic coating film on a steel sheet which does not pass through coating steps using coaters.
In electrodeposition, a polymer and a pigment are migrated to and deposited on a steel sheet under high voltage in an aqueous electrodeposition solution or dispersion containing a polymer (or resin) and a pigment. The uniformity of the film coated by the electrodeposition is inferior to that obtained by using a coater. Thus, a uniform thin film having a thickness of several .mu.m can be barely deposited on the steel sheet. Further, the use of high voltage power and an inevitable baking step after electrodeposition causes high production costs.
In electrolytic polymerization as an alternative coating method, a monomer is electrolytically polymerized to form an organic coating film on a substrate. The polymerization can be carried out under relatively low voltage. Electrolytic polymerization has been applied to electronic materials, such as capacitors, as disclosed in Japanese Examined Patent Publication Nos. 91-65,008, 91-61,314, and 92-7,521, in which the coated film primarily requires electroconductivity, and thus none of these references refer to achievement of corrosion resistance of the coated substrate.
Surface treatment for directly depositing an electrolytic polymer film on a metallic substrate is disclosed in Japanese Examined Patent Publication No. 75-15,485 and Japanese Unexamined Patent Publication No. 80-16,075, in which only an electrolytically polymerized coating film is formed on an aluminum or iron sheet, and no chromium layer is deposited. Therefore, the sheet after coating exhibits insufficient corrosion resistance at poor adhesion portions in electrodeposition performed after the electrolytic polymerization. When forming a thicker coating film, spot weldability of steel sheets deteriorates to an impractical level for use in vehicles.